Although the use of calcium hydroxybenzyl phosphonate for catalyzing the esterification of rosin with polyols is known to improve the reaction rate as well as the oxidative stability and color, the ester products suffer from a lack of viscosity stability. Viscosity stability is especially important when the rosin esters are used as tackifiers in hot melt adhesives. It has been shown that an increase in viscosity of 25-30% occurs when a typical formulation comprising a 1/1/1 mixture of ethylene/vinyl acetate//wax//rosin ester is held at 177.degree. C. for 96 hours. While a change in viscosity of + or -10% might be tolerated, an increase in viscosity of 25-30% causes problems with movement of the adhesive through the processing equipment.
U.S. Pat. No. 4,725,384 discloses a process for preparing rosin esters used as tackifiers for hot melt adhesive formulations that are said to have improved clarity and viscosity stability. A combination of phosphinic acid and a phenol sulfide is used as the esterification catalyst and a magnesium salt of an organic acid is added to neutralize the acidity of the esterified rosin. However, the "improved" composition still shows a 26% change in viscosity after aging at 175.degree. C. for 72 hours.